It is known that a bulk acoustic-wave (BAW) device is, in general, comprised of a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. The fundamental resonance occurs when the wavelength of the mechanical wave is about twice the thickness of the piezoelectric layer. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is the predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonance frequency is increased. BAW devices have traditionally been fabricated on sheets of quartz crystals. In general, it is difficult to achieve a device of high resonance frequency using this fabrication method. When fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonance frequency to the 0.5-10 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or FBARs. There are primarily two types of FBARs, namely, BAW resonators and stacked crystal filters (SCFs). An SCF usually has two or more piezoelectric layers and three or more electrodes, with some electrodes being grounded. The difference between these two types of devices lies mainly in their structure. FBARs are usually used in combination to produce passband or stopband filters. The combination of one series FBAR and one parallel, or shunt, FBAR makes up one section of the so-called ladder filter. The description of ladder filters can be found, for example, in Ella (U.S. Pat. No. 6,081,171). As disclosed in Ella, an FBAR-based device may have one or more protective layers commonly referred to as the passivation layers. A typical FBAR-based device is shown in FIGS. 1a to 1d. As shown in FIGS. 1a to 1d, the FBAR device comprises a substrate 501, a bottom electrode 507, a piezoelectric layer 509, and a top electrode 511. The electrodes and the piezoelectric layer form an acoustic resonator. The FBAR device may additionally include a membrane layer 505. As shown in FIG. 1a, an etched hole 503 is made on the substrate 501 to provide an air interface, separating the resonator from the substrate 501. Alternatively, an etched pit 502 is provided on the substrate 501, as shown in FIG. 1b. It is also possible to provide a sacrificial layer 506 separating the resonator and the substrate, as shown in FIG. 1c. It is also possible to form an acoustic mirror 521 between the bottom electrode 507 and the substrate 501 for reflecting the acoustic wave back to the resonator. The substrate can be made from silicon (Si), silicon dioxide (SiO2), Gallium Arsenide (GaAs), glass or ceramic materials. The bottom electrode and top electrode can be made from gold (Au), molybdenum (Mo), tungsten (W), copper (Cu), nickel (Ni), titanium (Ti), Niobium (Nb), silver (Ag), tantalum (Ta), cobalt (Co), aluminum (Al) or a combination of these metals, such as tungsten and aluminum. The piezoelectric layer 130 can be made from zinc oxide (ZnO), zinc sulfide (ZnS), aluminum nitride (AlN), lithium tantalate (LiTaO3) or other members of the so-called lead lanthanum zirconate titanate family. Additionally, a passivation layer typically made from a dielectric material, such as SiO2, Si3N4, or polyimide, is used to serve as an electrical insulator and to protect the piezoelectric layer. It should be noted that the sacrificial layer 506 in a bridge-type BAW device, as shown in FIG. 1c, is, in general, etched away in the final fabrication stages to create an air interface beneath the device. In a mirror-type BAW device, as shown in FIG. 1d, the acoustic mirror 521 consists of several layer pairs of high and low acoustic impedance materials, usually a quarter-wave thick. The bridge-type and the mirror-type BAW devices are known in the art.
The disadvantages of the membrane type approach are that it is difficult to produce the layers 507, 509, 511 on top of the membrane 505 so that they have sufficiently small mechanical stress, which would break or bend the membrane 505. Moreover, the membrane structure is not very rugged mechanically, which complicates the handling and dicing of the wafers. The mirror structure is clearly more rugged because the whole structure is solidly mounted on the substrate 501. As such, the mirror structure provides better thermal relief to the substrate in high power applications.
The mirror operates basically as a λ/4 transformer in that it consists of multiple pairs of alternating layers with high and low acoustic impedance, each of which is acoustically about one quarter wavelength thick. Thus, the whole stack transforms the impedance of the substrate to a very low impedance at the mirror/bottom electrode interface, creating an acoustically reflective interface similar to the air-interface in membrane type structures. The optimal operation of the mirror requires that the difference in the high and low impedance is as large as possible. In a mirror type BAW, the air-interface and the acoustic mirror form a cavity therebetween for confining the acoustic energy in an acoustic resonance, as shown in FIG. 2. The equivalent circuit of anyone of the BAW resonators mentioned-above is shown in FIG. 3. The equivalent circuit includes an equivalent resistance (R), an equivalent inductor (Lm), an equivalent capacitance (Cm), and a parallel parasitic capacitance (Co). As such, BAW resonators can be used as building blocks in impedance elements filters, such as ladder and lattice filters. Both ladder and lattice filters are known in the art. For example, ladder filters are disclosed in Ella. The basic unit in a ladder filter is an L-section 600, as shown in FIG. 4a. The L-section 600 includes two BAW resonators, a series resonator 500a and a shunt resonator 500b. When the L-section is used as a bandpass filter, the resonance frequency of the shunt resonator is designed to be slightly lower than that of the series resonator. The idea is to have the parallel resonance (=ideally infinite impedance) of the shunt resonator and the series resonance of the series resonator (=ideally zero impedance) at or close to the center frequency of the passband. The equivalent circuit for the L-section 600 is shown in FIG. 4b. The frequency response of a typical bandpass filter consisting of several (in this case 3) L-sections 600 is shown in FIG. 5. As shown in FIG. 5, the frequency response has a passband section centered about 940 MHz. In the response as shown in FIG. 5, the notch below the passband is due to the series resonance of the shunt resonator (signal being effectively grounded), and the notch above the passband is caused by the parallel resonance of the parallel resonator (signal seeing an infinite series resistance). The passband characteristics of BAW resonator combination can be improved by using two or more L-sections. The number of L-sections mostly influences the amount of attenuation outside the passband. It should be noted that ladder filters are usually referred to as having complete stages. Those ladder filters include equal number of series and shunt resonators. However, a ladder filter can have, for example, 3 shunt resonators and two series resonators (referred to as 2.5 stages).
In addition to using L-sections as passband filters, it is known in the art to make band-reject filters by switching the roles of series and shunt resonators. In this case, the series-connected resonators (500a, 500c) have their parallel resonance and the shunt resonators (500b, 500d) their series resonance at the intended stop-band, so as to provide a deep and steep notch at this frequency. The topology of such a filter with 2 L-sections is shown in FIG. 6, and the typical response of such filter is shown in FIG. 7. The response is, in principle, a mirror image of the passband ladder filter (see FIG. 5), in that the minimum insertion loss occurs at the “notch” frequencies below and above the stop band. The filter in such a format is, however, not very useful because the insertion loss outside the designated reject frequencies is still very large, typically from 3 to 6 dB, depending on the design. Accordingly, such a filter is not very useful as a band-reject filter in a multiband, double-mode mobile phone engine front-end.